High resolution NMR spectroscopy requires a B.sub.o magnetic field variation across the volume being observed of less than 1 part in 10.sup.7. As the sample itself will introduce perturbation to the field, this degree of homogeneity is normally achieved by adjustment of the B.sub.o field with each sample. This process is known as "shimming" and involves adjusting the currents in as many as 18 electromagnets, each of which has a very specific contour. In volume-selected NMR spectroscopy it is normal practice to shim the whole volume using a simple pulse and acquire sequence before attempting to perform the volume selected experiment. As this procedure is an iterative process, it can be very time consuming. In the traditional high-resolution experiment the homogeneity adjustments are made using the response from the whole sample which is carefully positioned about the magnet isocenter. By comparison, for in vivo spectroscopy, the whole sample is large whereas the volume of interest is relatively small and not in general positioned at the magnet isocenter. The adjustment of the magnetic field homogeneity may now be difficult. It is only at the magnet isocenter that shim sets are not strongly coupled. This is clearly a serious consideration when high-resolution spectra are required from sites remote from the isocenter.
The major goal of magnetic resonance spectroscopy (MRS) is to obtain a high-resolution spectrum from a known position. This task appears to be best achieved using image-directed spectroscopy whereby one of a variety of gradient-encoded volume-selection techniques is implemented using excitation frequencies determined from a proton density image of the object. With the notable exception of the VOSY method referred to herein, no volume-selection technique achieves spatial selection in one acquisition and, in general, eight acquisitions are required to complete the volume-selection procedure. This add-substract cycle often places severe demands upon the preamplififer and ADC. In the present inventor's experience, however, the major experimental difficulty relates to the adjustment of the shim set for high-resolution acquisition from the voxel of interest.
Since, over small distances, the inhomogeneities approximate to a straight line, only the three first-order shim gradients (X, Y, Z) need be used to correct the homogeneity of the B.sub.o field. In addition, the spectral response from spins outside the voxel of interest may be severely broadened by the shimming procedure, thus reducing the dynamic range requirements of the preamplifier and ADC.
For this concept to be exploited it is essential that the volume selection technique which is used performs complete suppression of signals arising from outside the volume of interest in a single acquisition. Thus, in medium-bore and whole-body magnets, it should be possible to achieve high-resolution shimming on a small voxel using only the X, Y and Z shims.